At present there are two publicly available GPS systems, known as NAVSTAR, owned by the USA, and GLONASS owned by the Russian Federation. These have been in existence for around two decades, but in the near future it is hoped that the European regional augmentation of GPS will start to provide its services, followed within a few years by a European system under the name of GALILEO.
The existing systems have been progressively refined so that using a differential phase implementation a locational accuracy of less than 2 cm can potentially be achieved over a baseline of 1000 km, but with a cost in computation and in the time taken to determine the location. Real time or near real time measurements have a correspondingly lower resolution, and at present the requirements for high precision mean that additional augmentations are necessarily employed to supplement the GNSS information. Furthermore, these could include a receiver taking measurements from many satellites, up to all those visible to it whereby to calculate an over-determined position solution and rejecting inconsistent data to improve the accuracy of the position solution. Such a system may use data from more than one constellation of GNSS satellites, GPS and GLONASS.
Although GNSS is used mainly for establishing the location of a user having a suitable, usually mobile, receiver, it is also used in respect of providing accurate time signals to users whose locations are already known or do not need to know. Single user position determining sets have simple receivers of satellite transmissions and circuitry that effects modelling of at least some atmospheric effects that influence signal reception so as to go some way towards eliminating errors in calculated position.
However, whether the user is interested in obtaining a position or a time measurement, a significant error arises from the inability to model accurately the delay to the GNSS signals caused by the atmosphere, namely the ionosphere and troposphere.
Satellite navigation users generate their three-dimensional position and time solution by processing four 4 or more pseudorange measurement to four or more satellites. A pseudorange measurement is the difference between the satellite clock time at signal broadcast and user receiver clock time at reception. The pseudorange observation is therefore related to the radio propagation time and therefore range between satellite and user. As estimates of the satellite position are known (they are broadcast by the satellite) a user can solve for the four unknowns (three-dimensional position and time) using four or more pseudorange observations. As part of the user's navigation/time solution filter pseudorange observations are corrected for variations in radio propagation time from that of free-space propagation.
In the user's navigation/time solution filter, a number of corrections are applied to the raw pseudorange measurement including tropospheric, ionospheric and relativistic corrections.
It has been suggested in WO-A1-03/069366 how to accommodate ionospheric delays and by use of a so-called server site which receives GNSS satellite signals, derives correction factors applicable to GPS receivers in the vicinity before broadcasting them locally so as to be received by such a GPS receiver and used to modify the on-board model used to correct such delays. For ionospheric delays, which comprise a small degree of signal path refraction and a more significant change in signal velocity, the delays and corrections therefor are substantially constant over a period of time that requires updating of correction data at most a few times per day.
Tropospheric effects on the other hand are relatively fast changing (or short-lived) and geographically localised, resulting primarily from weather or meteorological phenomena rather than climatic phenomena. However, the troposphere constitutes one of the largest identified sources of error in the effect that it has on signals propagating therethrough. The troposphere introduces ray bending and therefore an increase in signal path that constitutes a signal delay which is influenced by a number of meteorological factors, but particularly water content. Tropospheric delays are difficult to model simply.
Traditionally, tropospheric delay has been handled by the use of global tropospheric delay models that work from so-called climate parameters that relatively invariant and can be stored in the user receiver, but these parameters at best constitute an average or seasonal expectation, but not one that is meteorologically based, that is, based upon current, recent or predicted weather conditions.
One such model that is used and may be built into a portable GPS receiver is the RTCA tropospheric zenith delay model for WAAS users described in “Minimum Operational Performance Standards for Global Positioning Systems/Wide Area Augmentation System Airborne Equipment” RTCA DO229C, November 2001.
Such model is useful insofar as it simplifies tropospheric delays to zenith values (identified herein as DZ or ZTD) but there is still the need to map these for elevation effects caused by low satellite inclinations to the user. One such mapping model is described by Niell in “Global mapping functions for the atmosphere delay at radio wavelengths” Journal of Geophysical Research Vol 101, No B2, Pages 3227-3246, February 1996.
However, although these models permit incorporation into a user receiver they are inherently limited in ability to accommodate changes in tropospheric conditions that affect signal delays caused by the constantly changing, and localised weather.
Although models exist for deriving accurate tropospheric data by taking into account the meteorological conditions in one or more regions, such as by numerical weather prediction (NWP), the localised nature and thus large amount of data generated has been perceived as confirming that presently they cannot be used to sensibly improve upon practicable devices; that is, due both to this data being too large to be sent over communication systems that are available to mobile users and the limited capacity for processing within a reasonable amount of time.
The present invention provides a method of obtaining tropospheric delay data for use in a satellite positioning system or GNSS comprising the steps of generating for a user location, at a location remote from the user location and from meteorological information, at least one accurate tropospheric delay value, applicable to the user location for communication as a tropospheric delay correction to a said user.
Preferably said accurate tropospheric delay values are derived by a ray tracing technique. The accurate tropospheric delay values may be derived by three-dimensional refractive index field generation. Furthermore, it is also preferred that said meteorological information is based on numerical weather prediction (NWP) data. The meteorological model or each said tropospheric delay value correction derived therefor may be augmented by directly observed meteorological data.
In an embodiment of the invention, applicable to a user whose position is not accurately known, the method may comprise generating, from a first model which is known per se, a first set of approximate tropospheric delay values applicable to various user geographical locations, generating from a meteorological model employing such meteorological information a second set of tropospheric delay values that are accurate and applicable to said various user geographical locations, developing a set of delay value modifications for use with said first model so that it can provide a set of tropospheric delay values substantially in agreement with the second set, and expressing the set of modifications as a set of tropospheric delay corrections for communication to a said user.
The first model is based on non-meteorological parameters, which parameters comprise at least one of time of year, latitude and altitude. The non-meteorological parameters may further comprise at least one of longitude and time of day.
In the method of this embodiment the first and meteorological models develop sets of tropospheric delay values comprising zenith tropospheric delays. The first model may contain a mapping function relating tropospheric delay at a given elevation angle to the zenith tropospheric delay, and said set of delay value modifications may comprise a set of modifications for use with the mapping function of the first model.
Preferably, the modifications to the delay values are the differences between corresponding values of the sets attributable to the first and meteorological models. The corrections to be communicated may be the modifications per se or, preferably, the modifications expressed as a fractional change from the values of the first set, for example as a percentage.
Thus a correction may be effected as an addition or multiplier to any value generated by use of the first model.
The accurate tropospheric delay values are derived by a ray tracing technique, to determine the path of the satellite signals through the troposphere to the user and hence estimate the delay from a direct path, and possibly employing three-dimensional refractive index field generation.
It is possible to effect comparable corrections to mapping functions in such a first model that also rely upon paths affected by tropospheric delay.
The meteorological model may be based on numerical weather prediction (NWP) data for a region of the earth or real time meteorological data or both. In particular, the meteorological model or each said tropospheric delay value correction may be augmented by directly observed meteorological data, such as available in two-dimensional form from some imaging satellites.
The region for which data values are obtained may be substantially global or may be a smaller region as defined in NWP schemes as mesoscale maps.
In both cases it is possible to generate a set of (zenith) tropospheric delay values for each of a grid of locations over a region, as a two-dimensional array defining the geographical points at each of which is a delay value. Thus it is possible to create from the meteorological model and non-meteorological model zenith tropospheric delay value modifications and a set of corrections as a data array having values determined for individual grid points on the earth's surface, and the set of values comprises a distribution of said modifications over at least part of the earth's surface.
Having regard to the nature of the delay values and corrections, data reduction may be applied to the correction set, deriving a reduced data set for communication to a user.
Preferably, insofar as the nature of the correction set produces a data file analogous to that of a greyscale image reduction of the data size of the correction set is accomplished by an image compression process, conveniently, but not essentially, by lossy data reduction such as according to a wavelet-based JPEG 2000 or cosine JPEG standard.
By effecting a suitable level of data reduction, it becomes possible to communicate the correction set data to a user over a communication or data channel of limited bandwidth.
Thus having created a correction set suitable for use by a remote user having a satellite signal receiver, in accordance with this embodiment of the invention at least part of the correction set may be communicated via at least one orbiting satellite by transmitting the correction set as a reduced data image file to a said satellite and re-transmitting at least part of the set to a user from a said orbiting satellite.
Preferably this is achieved by communicating the corrections to at least one orbiting GNSS satellite from which user receives signals to establish at least one of position and time. In order to reduce even further the amount of data to be transmitted or re-transmitted, the method envisages communicating the image data to a said satellite for re-transmission of only that part of the correction data that can be of use to a user in a region within range of said satellite. This may be achieved by transmitting only said part of the correction data to the satellite or transmitting all of the data but causing the satellite to re-transmit only said part.
Insofar as such a satellite has limited capacity to transmit signals additional to those already transmitted and is in general only able to transmit any data, including correction data, periodically, it must be borne in mind that the meteorological environment is changing continuously as weather features vary their position in relation to the mapped region. Thus in addition to deriving tropospheric delay values associated with grid points of the mapped region it is necessary to apply data reduction sufficient to permit transmission of all or part of a said corrections useable by a user within a time, dictated by transmission availability and transmission rate of the satellite, substantially lower than the validity time of the meteorological information used by the meteorological model.
To ensure the validity of the tropospherically derived data, it is preferred to transmit said delay value corrections to a user corresponding to a meteorological temporal resolution of said meteorological model information of no greater than 1 hour, and/or corresponding to a meteorological spatial resolution of said meteorological model information of no greater than 90 km, insofar as time and distance are linked by speed of movement of relevant weather features.
By the above outlined image compression technique it is possible to effect correction data transmission to a user at a data rate in the range 25 to 500 bits/s and by selectively transmitting only parts of a global image applicable to a user in a relatively small region thereof, to permit correction data transmission effectively at rates well below the top of the range.
Insofar as the meteorological model derives a tropospheric delay values from data employed elsewhere to forecast or predict weather conditions at one or more locations, that is, conditions which vary with time, it is possible to predict tropospheric delay values in the future from said meteorological information and develop a prediction set of said corrections for a geographic region of the earth's surface, whereby each member of said prediction set describes a correction that becomes current as a function of time from development. It is therefore possible to communicate said prediction set of corrections as a batch and use members of the set as the time for which each was predicted becomes current in respect of the forecast.
Such communication may be to an orbiting satellite and the members re-transmitted one at a time as the time for which each was predicted becomes current in respect of the forecast.
A second embodiment employing the method is applicable when the position of the user receiver with respect to the server and/or GNSS satellites is known. That information may be employed by the server with the meteorological information to derive actual or mapped tropospheric delay values (rather than zenith delay values) for communication to the user for the purpose of setting or correcting the user receiver pseudoranging and obtaining accurate timing values. Such communication may be direct or via a network. It may also take place via one or more satellites, such as the GNSS satellites as discussed above, although data reduction may be required. Insofar as the users location is known, it is not expected to be necessary to derive and communicate a set of delay value corrections representing a distribution over a region. However, as discussed above, it may be appropriate to forecast weather conditions for any user location the user may be in and derive a predicted set of delay corrections and communicate these in batch form for use by the user receiver in turn as the time for which each member was predicted becomes current.
According to a second aspect of the present invention there is provided apparatus for obtaining data for use by a user of a satellite positioning system or GNSS, comprising generating means for generating, at a server location remote from the user from meteorological information, at least one accurate tropospheric delay value applicable to the user location and means to communicate at least a function of a said value to the user as a tropospheric delay correction.
The server may be arranged to derive a set of tropospheric delay values applicable to a plurality of user locations.
In a first embodiment, the apparatus of the preceding paragraph comprises first generating means for generating a first set of approximate tropospheric delay values from a first model which is known per se, second generating means for generating a second set of more accurate tropospheric delay values from a said meteorological model based on meteorological information, and developing means for developing from said first and second delay sets a set of tropospheric delay value modifications for use with said first model so that it can provide a set of tropospheric delay values substantially in agreement with the second set, and said developing means being arranged to express the modifications as a set of tropospheric delay corrections.
Preferably said first generating means utilises a said first model is based on non-meteorological parameters. Also, the developing means may be arranged to express said set of corrections each as a difference between corresponding values of the first and second sets, possibly as a fractional change from the values to be corrected.
The developing means is arranged to express the corrections as a distribution over a region of the earth's surface, preferably in the form of a data file corresponding to a greyscale image of multi-bit words, each word representing a location of the region. Furthermore the apparatus may include means for compressing said set of corrections. This may effect lossless compression of the set or for greater reduction, lossy compression on the set.
Each of the first and second generating means may advantageously derive corrections for parameters of at least an elevation mapping function used to map the zenith delay values to actual delay values. Corrections may be superimposed on the zenith delay correction data set as longer words for communication to the receiver
The apparatus also transmission means for transmitting said set of corrections to a user, and preferably to transmit via an orbiting satellite, which may be a satellite of the GNSS.
In a second embodiment, applicable to apparatus in which the position of a user receiver of satellite signals is known, apparatus is arranged to receive from the user information defining at least one of the user location with respect to the server or with respect to the GNSS satellites and to provide corrections in the form of tropospheric delay values per se rather than zenith delay values, although the latter could be provided.
According to a third aspect of the invention a GNSS user receiver comprises means operable to generate from an on-board model from non-meteorological data a set of approximate tropospheric delay values applicable to identification signals received from a plurality of said satellites and from and delay values and identification signals received from a plurality of said satellites compute an approximate position of the receiver relative to the earth's surface or time, means operable to receive a set of corrections to said tropospheric delay values derivable from the model, said corrections being derived from meteorological data, means to effect modifications to said derived delay values in accordance with the corrections and means to compute the position or time with greater accuracy.
Said means to effect modification to said delay values may be operable to effect interpolation or extrapolation of said corrections according to computed position of the user relative to locations for which the corrections have been derived
According to a fourth aspect of the invention a GNSS including a plurality of orbiting satellites, apparatus as defined above for obtaining data and a user receiver.
In the above discussion, tropospheric delay values and zenith tropospheric delay values have been referred to without regard to their nature. Whereas it is possible to derive a single troposphere delay value for a particular position, it is more usual to derive it as a so-called “wet” delay and a “dry” or “hydrostatic” delay. Apart from circumstances where it is important to distinguish, in particular in respect of data reduction, in this specification, references to tropospheric delay or delays and their derivation is intended to be read as deriving values for each.